Wide-area coverage is achieved in a number of ways in today's systems. Examples of existing access point coverage solutions include high-gain antennas, higher-order receive diversity, transmitter coherent combining (TCC), and high-altitude antennas (up to hundreds of meters above ground).
All these solutions are based on adapting the access point configuration to enhance the coverage. On downlink, increased EIRP (Effective Isotropic Radiated Power), as provided by high-gain antennas and TCC, gives increased signal strength at the terminal (user equipment). On uplink, high-gain antennas and higher-order receive diversity provide an increase in effective access point receiver sensitivity by extracting additional signal energy using effectively larger antenna apertures. Finally, high-altitude antenna installations provide enhanced coverage (on downlink and uplink) by reducing the path loss.
The solutions outlined above provide increased coverage, but often with a marginal rate of return that approaches zero (or becomes small enough not to warrant implementation of the solution) when the desired improvement in coverage is significant, say tens of dBs or more in terms of signal strength. The drawbacks of some existing solutions, in terms of their providing major coverage improvements, are presented below.
High-gain antennas derive their high gain from a decrease in half-power beamwidth (which is directly related to the antenna size). However, the smallest useful beamwidth is limited by the angular spread of the propagation environment, which means that the effective installed antenna gain becomes significantly lower than that of the antenna in free space when the (free-space) beam is too narrow. The smallest useful beamwidth, in the elevation plane, is also limited by the expected sway profile of the tower or mast: the beam must be wide enough to maintain proper illumination of the desired coverage area within the interval of realized pointing directions that result from tower or mast movement. High-gain antennas can provide coverage improvement on both uplink and downlink.
Transmitter coherent combining (TCC) is based on using multiple (power) amplifiers in parallel and combining their output signals to generate an effective output power equal to the sum of the output powers of the individual amplifiers. One drawback of additional amplifiers is increased energy consumption related to running and cooling the amplifiers, which gives increased OPEX. TCC is a downlink-only method for coverage improvement.
Higher-order receive diversity works by extracting signal energy using multiple sensors (antennas) at different locations, in different directions, and/or with multiple polarizations. It requires an additional uplink radio chain (including antenna, tower-mounted amplifier, feeder, and radio) for each additional sensor, which gives increased capital expenditures. It also requires larger than conventional cabinets to accommodate the extra receiver equipment. Higher-order receive diversity is an uplink-only method.
The hitherto described methods for generating increased coverage (wide-area coverage) share the common drawback that a 3 dB increase of coverage requires a doubling of the “equipment”: the area of high-gain antennas must double for every additional 3 dB of gain, TCC requires twice as many amplifiers for a 3 dB gain, and twice as many receiver radio chains are needed to get a higher-order receive diversity gain of 3 dB (ignoring gain due to fading statistics, which approaches zero when the number of receiver chains is large). Obviously, there is a limit for any practical application at which the cost, i.e. capital expenditures (CAPEX) and/or operational expenditures (OPEX), and sheer volume and weight of the equipment make these types of coverage solutions unsuitable.
Yet another coverage method is high-altitude antennas which improve the path loss by providing line-of-sight propagation to a larger part of the coverage area, be it directly to terminals or to reflection/diffraction points in the environment. Because of the large distance to ground, the signal correlation over the antenna aperture may also be improved resulting in higher effective gain (approaching the free-space gain). However, high-altitude antennas require high masts or towers and may require long feeder cables. The former can make the total access point very expensive (CAPEX), whereas the latter can be both costly and inefficient due to transmission losses in the feeders (CAPEX and OPEX). High-altitude antennas can provide coverage improvement on both uplink and downlink.
In conclusion, present access point-based coverage solutions can provide improved coverage, but become increasingly inefficient as the coverage requirements are raised.
Traditional repeaters are also used to create coverage. However, a traditional repeater uses one single sub-node link antenna with a single main beam for communication with one specific access point. This can be a very poor solution in many systems. For example, in CDMA systems, a property called cell breathing is common. Cell breathing refers to the (slow) dynamic expansion and contraction of the footprint of a CDMA cell, which may depend on the number of users connected at any given moment or, in general, the traffic load in the cell and which can be used to balance the load between neighboring cells. Pro-active cell breathing (and cell optimization in general) can be achieved by for example tuning of pilot power and antenna tilt. Since a traditional repeater provides coverage for a fixed area, it defeats the purpose of pro-active cell breathing by always providing coverage over a particular area for the same access point. In addition, the quality of the communications link between access point and repeater is affected by the cell breathing, when cell breathing is performed using power control of cell-defining pilot signals or antenna tilt. In this case, the performance in the area covered by the repeater may show unacceptably large fluctuations.
An example of a prior art bidirectional repeater for wireless communication systems is disclosed in EP 1 445 876, assigned to California Amplifier Inc. The disclosed repeater is provided with a link antenna to establish communication with a dedicated base station (access point) and a bidirectional coverage antenna to generate coverage in a geographical area poorly covered, or not covered at all, by the base station coverage antenna. This type of repeater may be used in a communication system as shown in FIG. 1. One base station 2A is provided with an antenna system 3 that generates coverage to a geographical primary coverage area “A”. The same antenna system 3 (including transmitting and receiving antennas) communicates via signals 4 with a repeater 5a. The repeater 5a receives and transmits signals to the base station 2A using a link antenna 6, and generates coverage to a geographical secondary coverage area “a” using a coverage antenna 7. A terminal 8 communicates via signals 9 with the repeater 5a using the coverage antenna 7, and a communication link between the terminal 8 and the base station 2 is established through the secondary coverage area a provided by the repeater.
In U.S. Pat. No. 4,727,590, by Minori Kawano et al., a repeater is disclosed having a link antenna to establish communication with one or more dedicated base stations (access points) and a receiving antenna to receive signals in up-link from a terminal close to the repeater and direct the signals to the base station closest to the terminal. The terminal receives signal in down-link directly from the base station coverage antenna. This type of communication network is shown in FIG. 2 comprising three base stations 2A, 2B, 2C. Each base station is provided with an antenna system 3A, 3B, 3C that generates coverage to geographical primary coverage areas A, B and C, respectively. These primary coverage areas are normally overlapping, but a straight line is drawn for illustrating purposes. A repeater 5abc is arranged at a position with equal distance to the three base stations, and all three base stations communicate with the repeater independently of each other. The repeater 5abc is provided with receive coverage antennas to receive signals from terminals 8 close to the repeater 5abc, and transmit link antennas to communicate with the base station. Only secondary reception coverage areas a, b and c are thus generated by the repeater. In up-link, a terminal 8 arranged within secondary reception coverage area b transmits a signal to the repeater 5abc, and the repeater then forwards an amplified signal to the base station 2B. In down-link, the base station directly transmits a signal to the terminal 8.
A problem with the existing repeater stations, or relay stations, is the imperfect coverage performance in a cellular network. The existing cell plan, with its location of access points (base stations), cannot provide cell-wide coverage, at points within the desired coverage area, or at the border of the desired coverage area, or both.